Coating compositions which are capable of acid catalyzed crosslinking generally contain a catalytic amount of an acid catalyst. The acid catalyst is added in order to accelerate crosslinking and thereby reduce the overall time required for cure. When an acid catalyst is utilized, it is advantageous to have the catalyst present as the free acid since in this manner a rapid cure can be efficiently achieved. However, the presence of the free acid may also cause problems in storage stability; that is, the coating composition will exhibit a tendency to gel and harden during the storage term, thus becoming unfit for use.
As a means of circumventing these difficulties, latent or blocked acid catalysts are often utilized to delay the action of crosslinking agents and otherwise postpone the curing mechanism.
Latent acid catalysts are formed by preparing a derivative of an acid catalyst such as para-toluenesulfonic acid (pTSA) or other sulfonic acids. For example, a well-known group of blocked acid catalysts are amine salts of aromatic sulfonic acids, such as pyridinium para-toluenesulfonate. Such sulfonate salts are less active than the free acid in promoting crosslinking. During cure, the catalysts are activated by heating which results in liberation of the free sulfonic acid catalyst.
The compositions of U.S. Pat. Nos. 4,192,826 and 4,281,075 are exemplary of coating compositions which utilize a latent acid catalyst to delay the action of the crosslinking agent until cure. U.S. Pat. No. 4,192,826 is directed to thermosetting liquid coating compositions based on polymer capable of being crosslinked with nitrogen resin crosslinkers, and blocked acid catalyst prepared from materials containing at least one oxirane functionality and a sulfonic acid. The blocked acid catalyst resulting from these reactants is a beta-hydroxy sulfonic acid ester. U.S. Pat. No. 4,281,075 is directed to thermosetting compositions based on a film-forming component bearing or capable of generating hydroxyl functionality, amine-aldehyde crosslinking agent, and a latent acid catalyst having the formula: EQU R--(SO.sub.2)--O--X
wherein R is selected from alkyl or aryl groups and X is selected from 2-hydroxycycloalkyl and aryl substituted 2-hydroxycycloalkyl groups. The latent acid catalysts depicted by this formula are also beta-hydroxy sulfonic acid esters.
Also exemplary of coating compositions utilizing latent acid catalysts are GB Pat. Nos. 1,361,929 and 1,413,054.